A great variety of polymeric materials known as graft polymers have been prepared prior to this invention. Perhaps the best known graft polymers are those known as ABS graft polymers which are prepared by polymerizing a monomer such as butadiene or monomers such as butadiene and styrene to form either a homopolymer or a copolymer backbone substrate and thereafter polymerizing grafting monomers such as styrene and acrylonitrile in the presence of the prepolymerized backbone substrate material.
In preparing graft polymers, either a rubbery backbone or a resinous backbone may be utilized. If the prepolymerized monomer is predominately a rubber forming monomer such as a conjugated diene, the backbone will be flexible and if the monomer is predominantly a resin forming monomer such as styrene, the backbone material will be resinous and inflexible. The polymerizing of a second monomer or group of monomers in the presence of the prepolymerized backbone material causes the second monomer or monomers to become bonded in chains on the prepolymerized backbone to form what is known as a graft polymer.
The most conventional ABS graft polymers are those wherein the prepolymerized material is a rubbery polymer or copolymer such as polybutadiene or butadiene-styrene and those monomers that are normally copolymerized in the presence of the rubbery backbone are acrylonitrile, styrene and acrylic acid ester such as methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate and the like.
Generally, those monomers that were utilized as grafting monomers in the preparation of ABS graft polymers prior to this invention were monovinyl aromatic hydrocarbons, acrylonitrile, and substituted acrylonitrile. Though these materials exhibited high impact strengths and a favorable balance of physical properties, they were generally known to be opaque and therefore were difficult to load with a high degree of pigment and could not be used in applications where clarity was a desired characteristic.
Perhaps one of the greatest disadvantages in obtaining transparency with the previously known ABS graft polymers was their tendency to lose impact strength at a high degree of transparency. Further, as mentioned, it was difficult to load to opaque and translucent polymers to obtain depth of color while retaining the desired high impact characteristics. It was found that if the refractive index of the substrate did not closely coincide with that of the grafting monomers, the polymeric materials would necessarily be translucent or opaque.